1. Field of the Invention
The present invention relates to a method of producing paper webs coated on both sides and to an apparatus for the application of such method.
2. Description of the Related Art
More exactly, the present invention concerns a so-called multiple coating method in which a liquid medium is applied twice, successively on each paper side. Such multiple coating method is known from the magazine xe2x80x9cWochenblatt fxc3xcr Papierfabrikationxe2x80x9d No. 11/12, 1974, pages 339 through 400; refer specifically to FIG. 4 on page 396. Illustrated there are four coaters of essentially the same structure. In each of the coaters, the paper web runs over a backing roll, with liquid medium being applied directly onto the paper web. Known from FIG. 1 of the same publication, however, is also the following: A precoat can first be applied indirectly onto each paper side, followed by a top coating applied by a direct method. Refer to xe2x80x9cWochenblatt fxc3xcr Papierfabrikationxe2x80x9d No. 10, 1955, pages 436 through 443, notably FIG. 15.
Underlying the present invention, based on this prior art, is the improvement of the known multiple coating method and the apparatus provided for it, to the effect that several differently coated paper grades can be produced without appreciable modification of the machine. The present invention provides the ability to selectively produce so-called MWC (medium-weight catalog) paper or so-called LWC (lightweight catalog) paper. Understood as MWC and LWC papers are medium-weight and lightweight coated papers, respectively.
The present invention uses one or two initially provided applicators, with which liquid medium is applied indirectly onto the paper web, to apply, in the production of MWC paper, a so-called contour coating of pigmented coating color simultaneously or successively onto both paper sides. Alternatively, in the production of LWC paper, only one, preferably very thin, sizing suspension layer is applied simultaneously or successively onto both paper sides.
Hence, in the production of MWC papers, each web side is coated twice overall with coating mixture. In contrast, in the production of LWC papers, the indirectly applied sizing suspension forms on each web side a barrier layer between the raw paper web and the directly applied coating mixture.
Advantages of the present invention include, in the production of MWC paper, a considerably improved opacity as compared to the prior art, achieved with the aid of the applicator(s). In the production of LWC papers, the first two coaters, in contrast, make possible the use of a raw paper that is thinner than before. Also, a raw paper having a considerably lower coating weight, with the application of the sizing barrier layer, is able to have a top coating applied. The reason for this is that there is little or no penetration of the coating mixture into the inside of the raw paper. Therefore, a surface of high uniformity (better cover) is achieved, despite the low coating weight.
The metering device of the applicator(s) working indirectly may feature as a metering element a blade, a smooth roll doctor or a grooved roll doctor. The applicator of the coaters working directly may be configured as a roll applicator, a pressure chamber applicator (SDTAxe2x80x94short dwell time applicator), or as an open-jet applicator. The dryer units feature top-felted single-felt cylinder groups, since this configuration most easily allows a quick removal of broken parts, which is important for on-line operation.